1. Field of the Invention
The present invention relates to RF data links in which data is sent from one or more RF transmitters to one or more RF receivers in a data format utilizing data frames and blocks. More particularly, this invention pertains to methods of improving the quality of data sent across an RF data link as used in a remote tire pressure monitoring systems and the like.
2. Description of the Prior Art
Remote tire pressure monitoring systems (TPMS) are used in motor vehicles to provide accurate tire pressure information to the driver in real time. A typical TPMS will have a pressure sensor mounted in each wheel, with an RF transmitter coupled to each sensor. The transmitter receives pressure data from its corresponding sensor and sends the data to a receiver. The receiver is under the control of a processor which then decodes the data and communicates the tire pressure information to the driver.
Preferably, the RF data link between the transmitter and receiver is configured to send digital data. Thus, each RF signal is formatted as a data frame. Each frame of data transmitted by a wheel sensor/transmitter in a remote tire pressure monitoring system includes a sequence of bits sometimes referred to as a data record, word or character. The frame may contain information corresponding to the tire/wheel identifier which uniquely identifies the sensor/transmitter combination, and tire data, such as data corresponding to the measured tire pressure of the tire. This data is sometimes referred to as the “payload.” Other information, such as a frame header or synchronization data may be transmitted as part of the frame. Multiple frames of data forming a data block may be transmitted during respective time periods. For example, a data block may contain eight frames of data sent during eight time periods. Data blocks are preferably repeated in the TPMS at an update frequency. The update frequency may be selected to be on the order of seconds, minutes or hours, or any other suitable rate. Also, the update frequency may be varied depending on the mode of operation such as when the wheel is stationary or rolling.
A frame or block of data is conventionally subjected to some type of error detection by the processor in the TPMS. For example, a checksum step is a basic error-detection technique in which each transmitted frame or block includes a numerical value based on a number of set bits in the frame or block as determined by a checksum algorithm (e.g., CRC) in the transmitter. The receiver then applies the same checksum algorithm to the received frame or block to determine if the numerical value calculated from the received data matches the transmitted numerical value. If it does not, the processor can assume that the received data has been corrupted.
The RF data link in a TPMS must operate in a harsh environment that is electrically noisy. The strength of the transmitted signal can be degraded by ambient conditions. Accordingly, it is common for the transmitted data to be corrupted or garbled. Data bits can be dropped or incorrectly sent, resulting in invalid data, no data, and/or slow processing time at the receiving end. Certain invalid data is sometimes referred to as “implausible” because its value falls outside a range of what could reasonably be expected during normal operation of the system.
Attempts have been made in the prior art to improve the quality of the RF data link in TPMS. These have included hardware changes to either the TPMS wheel transmitter or TPMS receiver to improve the RF signal quality of the RF link itself. Other efforts have focused on providing more protection in the transmitter against spurious readings of temperature and pressure (EMC, ASIC filtering, etc). Unfortunately, these solutions have not been entirely satisfactory and any solution that adds hardware to the system will increase the cost of the system.